It is known to use free electron laser (FEL) radiation sources to produce radiation of a desired wavelength, in which an electron beam comprising a periodic sequence of electron bunches is passed through an undulator to generate the radiation. Such sources can be used to produce radiation in a range 4 nm to 25 nm, for example extreme ultra-violet (EUV) radiation, or at other desired wavelengths.
In known FEL radiation sources, ions are produced from residual gas in the electron beam through collisional ionization.
Known FEL sources include LINACs for accelerating (and decelerating) electron bunches before (and after) they pass through the undulator. Energy recovery LINACs can be used, which are usually designed to operate with a balanced cavity load close to zero (e.g. currents in accelerating and decelerating beams match, and energy extracted and deposited upon acceleration and deceleration almost match).
The sequence of electron bunches, which may be referred to as an electron bunch train, may comprise a sequence of electron bunches spaced apart in time, and having different energies and being at different stages of the acceleration and deceleration cycle. It is important that all bunches are precisely aligned in the LINAC or LINACs, both in a lateral direction (e.g. lateral position in a plane perpendicular to the direction of propagation) and longitudinal direction (e.g. separation between successive bunches in time or distance in a direction of propagation of the bunches).
Precise alignment of the bunches can be important to ensure that the electric field integrated over the path length is constant/stable for all bunch energy thereby to assure a well-defined energy of the generated radiation. A gradient in electron energy may be applied over the bunch, such that the electrons in front of the bunch have higher energy than the electrons at the end of the bunch, in view of the eventual bunch compression that may be performed using magnets, to ensure that the integrated field per electron for a given position in the bunch is constant/stable. Precise alignment of the bunches is also important as any deviation of the bunches from the centre of the LINAC will result in a kick due to a gradient in the magnetic field. Both effects may have a large impact on the propagation of the bunches and consequently the yield and stability of the generated radiation.
Beam position monitors are known, which can be used to determine lateral position of an electron beam or electron bunch sequence, or any other suitable charged particle beam. A known beam position monitor is based on capacitive pickup of the coulomb field of the traversing beam. Four electrodes can be spaced with an angular separation of 90° around the beam path. For each quadrant, an electrode picks up a signal. From the signals the 2-D lateral position of the beam can be reconstructed based on the charge induced on the electrode. The charge on the electrodes is read using read-out electronics. Since the falling time following performance of a measurement is slow and since reflections on the electrodes disturb the signal the difference in position between two adjacent bunches can be difficult or impossible to measure using such a known beam position monitor. Other beam position monitors are also known which use different types of electrodes and electrode geometries, for example with electrodes positioned with different angular separations.
Bunch arrival time monitors are also known. A known bunch arrival time monitor measures the time of arrival of a bunch with respect to, for instance, a master clock. In such a known monitor, dedicated electrodes may be coupled to electro-optic modulator crystals rather than the read-out electronics used for beam position monitors. Four such electrodes can be spaced with an angular separation of 90° around the beam path, with each pair of opposing electrodes being coupled to a respective electro-optic modulator crystal. Thus, two electro-optic modulator crystals are used to obtain measurements from the four electrodes, with each pair opposing electrodes being coupled to a respective one of the electro-optic crystals. The electric field measured by an electrode is a function of the proximity of the bunch to the electrode, although in many arrangements the signals of two opposing electrodes are combined to eliminate position dependence. The monitor may be configured to provide a fine readout channel with high bandwidth limited range and a coarse channel with lower bandwidth and large measurement range.
The arrival time of a bunch is measured using the electro-optic modulator crystals, which change their optical properties when an electric field is applied. The changing electric field changes the properties of the crystal. A pulsed femtosecond laser, which may in some cases be guided by an optical fibre, probes the crystals. The timing of the optical reference pulse is adjusted such that the pulses sample the pick-up signal at its zero crossing. At this operation point the inherent dependence of the arrival time measurement on the bunch charge is reduced. All subsequent electron bunches whose time of arrival deviates from this reference point cause an amplitude modulation of the sampling laser pulses. Other bunch arrival time monitors with different electrode types and arrangements, or including other components such as R.F. cavities are also known.
The position of bunches in FEL radiation sources can be precisely adjusted using bending magnets and combiners/spreaders. However, accurate adjustments would require accurate diagnostics to see whether the bunch train in the LINACs is aligned correctly. Typical known beam position monitors cannot distinguish between different energies and may integrate measurements over all bunches
It is an aim of the present invention to provide an improved or at least alternative apparatus and method for measuring at least one property of an electron bunch or other charged particle bunch, for example in a radiation source.